The need for better ground and space based telescope resolution has driven the manufacturing of larger diameters of primary mirrors of such telescopes. However, larger diameter primary mirrors result in the primary mirrors having additional weight and manufacturing problems. For example, because large solid mirror blanks weigh more, they require more time to cast and to anneal. The heavier mirror blanks also bend under their own weight, and are more difficult to maneuver in the factory. For space-based mirrors, the zero gravity back-out, for testing purposes, is smaller and therefore, simpler and more accurate for lightweight mirrors. Special mounting and supporting is required if accurate testing is to be achieved.
In contrast, lightweight mirror assemblies, fabricated from lightweight mirror blanks, have the advantage of increasing the stiffness-to-weight ratio, and therefore the frequency of the first resonant mode. The higher first resonant mode ensures lower coupling between system vibrations and mirror vibrations. Lighter mirror blanks also reduce the gravity sag and the gravity back-out, reducing measurement uncertainty of a 1-G test in a 1-G environment. Light weighted mirror blanks, as it is termed in the industry by those skilled in the art, make the finished mirror assembly more tolerant of spacecraft maneuvers, as well as increasing the mirror's stability. Lightweight mirrors assemblies also result in lighter payloads and smaller launch rockets.
Lightweight mirrors are the desired end product. U.S. Pat. No. 5,071,596 (hereinafter “Goela”), incorporated herein by reference, shows a two-layer open back design where a separate layer of silicon is needed for the optical surface. This layer is deposited after the second layer coheres the core structure to the first layer. The core is described as an “‘egg crate’ core from graphite ribs about 0.020 inch (0.5 mm) thick . . . . ” The diagrams and descriptions show the core to be thin and substantially perpendicular to the face sheet. In this disclosure, the thin ribs are completely encased in deposited material and are not subsequently removed or oxidized. Therefore, they become part of the support structure. During thermal variation, small differences in the coefficient of thermal expansion (CTE) between the core material and deposition material exert forces on the optical surface and thereby distort the mirror.
Additionally, U.S. Pat. No. 5,741,445 (hereinafter “Taylor”), incorporated herein by reference, shows a three-layer design. Depending upon the desired application of the mirror, this three-layer design may be preferred. As disclosed in this patent, this design uses interlocking silicon carbide (SiC) ribs forming straight walls and a closed back. The design is quite scalable and divides the mirror into smaller components. The design requires secondary machining between depositions to render surfaces co-planar (e.g. the case of a piano mirror). Referring to the patent's back plate 20, “An access hole 64 is provided for each cell of the cell structure 26.” Again, more secondary machining in SiC is required.
Sloping walls allow thinner depositions and better cohesion at the interface between supports. As described in Goela, “[a]nother consequence of CVD deposition is that the walls of the backstructure [sic.] are tapered with the thinnest SiC coating near the faceplate.” This description reveals a shortcoming as the strength of the bond is limited by the coating thickness.